If you are behind a proxy you may run into a problem with how the proxy is caching things. You may recieve the error message,
W: GPG error: http://ca.archive.ubuntu.com intrepid-updates Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com> W: You may want to run apt-get update to correct these problems
To fix that do the following,
sudo apt-get update -o Acquire::http::No-Cache=True sudo apt-get update
If that does not solve it try,
sudo apt-get update -o Acquire::BrokenProxy=true sudo apt-get update